Dimethylmaleate is a very versatile technical intermediate product which may be used, for example, as starting material for the copolymerisation with other vinyl compounds and as a precursor for pharmaceutical preparations and plant protective agents.
A known standard process for esterification is the reaction of an alcohol with an organic acid, usually in the presence of esterification catalysts which are generally inorganic acids or arylsulphonic acids. If the alcohols are low boiling, such as methanol or ethanol, the esterification reaction progresses only slowly, mainly due to the difficulty of removing the water of reaction from the mixture. It is for the same reason that incompletely reacted reaction mixtures usually containing high proportions of monoesters are obtained in the esterification of dicarboxylic acids.
It has frequently been attempted to overcome these difficulties by using a very large excess of alcohol but this lowers the reaction temperature and reaction velocity to an extent which is economically undesirable. It is also customary to use water-carrying auxiliary substances in esterifications, but these are difficult to use in the special case of methanol as its boiling point is too low.
A process for the continuous production of esters is described In British Patent Specification No. 732,784. In this process, numerous heating devices are used to maintain the esterification mixture at a temperature at which fractionation of the reactants will be substantially avoided.
British Patent Specification No. 1,430,069 discloses a process which overcomes the above-mentioned difficulties for a large number of high boiling carboxylic acids and in which esters of high boiling organic acids with readily volatile alcohols are prepared in the presence of an esterification catalyst by introducing an excess of the alcohol into a solvent-free carboxylic acid melt and removing the water of reaction from the reaction mixture in the form of steam until the reaction mixture has substantially completely esterified and is substantially free from water and alcohol. This single stage, batchwise process which provides conversion rates in the region of 90%, based on the quantity of acid, is carried out at high temperatures which in the case of dimethylmaleate, for example, results in extensive transposition reactions to form dimethylfumarate, which is not wanted as reaction product.
By a more recent process described in German Patent Specification No. 3,114,320, maleic acid esters of short chained alcohols are also available. In this process, maleic acid, maleic acid anhydride or mixtures thereof with an excess of up to 300% of a readily volatile alcohol are reacted together under pressure to form dialkylmaleate. If the reaction temperatures employed are from 60 to 100 deg. Cent. above the boiling point of the alcohol put into the process, times of up to 19 hours are required for complete esterification. Under these conditions, the crude dialkylmaleate obtained has a fumaric ester content of up to 4% by weight, which is undesirable for many applications.
A mixture of 96% by weight of maleic acid dimethylester and 4% by weight of fumaric acid dimethylester, for example, has a melting point of about 25.degree. C. and is therefore technically more difficult to handle than pure dimethylmaleate (storage, effort required for pumping). Furthermore, these fumaric ester components may lead to undesirable side products as a result of secondary reactions which is a particularly serious disadvantage in view of the great importance of this maleic acid ester as intermediate product.